Google Adsense is changing the counting of impressions from served impressions to downloaded impressions. Google says that “as a result, some AdSense users might see decreases in their impression counts, and corresponding improvements in their impression RPM, CTR, and other impression-based metrics. Earnings, however, should not be impacted.”
The impression counting was updated on all Doubleclick products in January, this year, in compliance with the Media Rating Council (MRC). Now was Google Adsense.
The difference between Served Impression and Downloaded Impression: Served impression: An impression is counted when an ad is sent (or “served”) to a publisher by the ad server. The ad creative may or may not be downloaded to the user’s device, rendered, or viewed by the user. Downloaded impression: An impression is counted only after the ad has begun to download to the user’s device.